The present disclosure describes methods and systems, including computer-implemented methods, computer program products, and computer systems for direct gas reservoir detection using frequency slope. One computer-implemented method includes receiving seismic data corresponding to a target formation. Further, the method includes based on a time-frequency analysis of the seismic data, generating a representation of a time-variant frequency response of the seismic data. Yet further, the method includes generating a frequency spectrum for each of one or more locations within the target formation using the representation of the time-variant frequency response. Additionally, the method includes calculating one or more frequency slopes between a peak frequency and a maximum frequency of each frequency spectrum. The method also includes based on the one or more frequency slopes, determining for each location whether gas are present.